Need help with rigid pipe and bathroom fan/light

I'm fixing up a "renovated" bathroom done by a previous owner, and I've just dropped the fan/light to examine the wiring. Here is what was done: the old rigid (the house is from 1918) running to a ceiling light was cut, and then flexible conduit was stuffed into it to connect the existing rigid to the fan/light unit.

My question is this--are there any approved connectors for rigid pipe, such as compression fittings, that I could use to make a solid connection (rigid to a junction box, or rigid to flexible conduit)?

I've rewired up to the box that feeds the fan/light, and I don't think there is any way to budge the old rigid to get new EMT or flexible conduit in (it is flush to the joists above). To make matters more complex, all four knockouts in the feeding octagon box are being used. I don't think I could get a threading tool onto the cut end of the pipe to cut new threads and add a junction box.

Any ideas would be greatly appreciated. I am not against breaking into the walls if this is the only way to solve the problem properly.

Any way you could tear out the old mess and put in new wiring? I'll bet that old World War One era wiring is a little degraded, and that even if it's in perfect shape it probably wouldn't meet current code. That "all knockouts in the box" are in use makes me wonder if there's too many conductors and other things in that junction box.

Although I'm not sure what this "rigid" tubing is, I know current NEC allows for wires to exit a conduit if a bushing is installed to prevent mechanical stress and protect NM cable. That may not be what you're looking for, though. Check with Joe (the moderator) for a more specific way to approach this.

I'll bet that tearing out and redoing as much as you can per current code is an alternative you may feel compelled to settle for. Easy solutions are few and hard to come by.

The rigid is your standard steel pipe that was used all around Chicago early in the century. Like gas pipe--stiff and unruly, but quite durable.

If there are connectors for rigid pipe that don't have to be threaded on, this would be the most efficient fix. If such connectors don't exist, then I guess I'll have to get the rigid out somehow and make way for EMT or flexible conduit.

Box fill is not an issue (but thanks for raising the question). I replaced the feeding box (an old pancake box) with a deep octagon box, so the fill is fine (one conductor in, three branching out to other ceiling fixtures).

I don't have images of the connections, but they are quite standard. The connection to the hallway octagon box (feed) is the original rigid pipe (threaded with locknut), and the flexible conduit connects to the fan housing via a standard connector. It is this middle part that is so bad, the flexible conduit just stuck into the rigid.